[PATCH] MTRR: Fix race causing set_mtrr to go into infinite loop

[Loic Prylli <loic@myri.com>]

Processors synchronization in set_mtrr requires the .gate field
to be set after .count field is properly initialized. Without an explicit
barrier, the compiler was reordering those memory stores. That was sometimes
causing a processor (in ipi_handler) to see the .gate change and
decrement .count before the latter is set by set_mtrr() (which
then hangs in a infinite loop with irqs disabled).

Signed-off-by: Loic Prylli <loic@myri.com>
---
 arch/i386/kernel/cpu/mtrr/main.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/i386/kernel/cpu/mtrr/main.c b/arch/i386/kernel/cpu/mtrr/main.c
index 55b0051..75dc6d5 100644
--- a/arch/i386/kernel/cpu/mtrr/main.c
+++ b/arch/i386/kernel/cpu/mtrr/main.c
@@ -229,6 +229,8 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 	data.smp_size = size;
 	data.smp_type = type;
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	/* make sure data.count is visible before unleashing other CPUs */
+	smp_wmb();
 	atomic_set(&data.gate,0);
 
 	/*  Start the ball rolling on other CPUs  */
@@ -242,6 +244,7 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 
 	/* ok, reset count and toggle gate */
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	smp_wmb();
 	atomic_set(&data.gate,1);
 
 	/* do our MTRR business */
@@ -260,6 +263,7 @@ static void set_mtrr(unsigned int reg, unsigned long base,
 		cpu_relax();
 
 	atomic_set(&data.count, num_booting_cpus() - 1);
+	smp_wmb();
 	atomic_set(&data.gate,0);
 
 	/*
-- 1.5.2.2